A gyrometer is a motion sensor which makes it possible to measure the rotation rate of the reference frame of the sensor with respect to a Galilean reference frame, about one or more axes.
A laser gyrometer is a bidirectional ring laser which makes it possible to measure an angular rate or a relative angular position by temporal integration. It consists of an optical cavity composed of several mirrors assembled on a block in which paths are drilled to provide an optical cavity. An amplifying medium is inserted into the optical path of the cavity and an excitation system provides it with energy making it possible to generate the laser gain. The elements from which the laser cavity is composed are chosen so as to allow bidirectional operation: the laser cavity must be able to simultaneously sustain two waves which propagate in opposite directions (so-called counter-rotating waves).
The majority of laser gyrometers, also called laser gyros, use a gaseous amplifying medium which is customarily a mixture of helium and neon. However, it is possible to use a laser gyro with solid-state amplifying medium, in which the gaseous amplifying medium is replaced with a solid element, for example a Neodymium-doped YAG (Yttrium-Aluminum-Garnet) matrix.
The operating principle of a laser gyro is based on the Sagnac effect in a ring laser cavity to which a rotational motion is imparted. When the cavity is stationary, the two counter-rotating waves exhibit the same optical frequency. In the presence of a rotational motion in the plane of the optical cavity, the Sagnac effect induces a difference of frequency Ω between the two counter-rotating optical waves. A fraction of the energy of each wave is extracted from the cavity. A recombination device causes the two extracted beams to interfere so as to form interference fringes which are observed with the aid of one or more photo-detectors. In an ideal laser gyrometer, the frequency of the fringes in front of the photo-detector is proportional to the rotation rate impressed on the cavity and their direction of travel depends on the direction of rotation. In the solid media customarily used, including Nd:YAG, the two counter-rotating waves share the same amplifying atoms. One then speaks of homogeneous gain. Along the cavity, the two counter-rotating waves form a standing wave with intensity antinodes and nodes. When the cavity is at rest or rotating at very low rate, the interference grating remains stationary with respect to the cavity; when the cavity is subjected to a sufficient rate of rotation, the frequencies of the counter-rotating modes move further apart and the grating moves with respect to the elements of the cavity. The atoms of the gain medium participate all the more in the process of stimulated emission the closer they are to an antinode of the standing wave and all the less the closer they are to a node. A population inversion grating, registered by the intensity grating of the standing wave, is then created in the gain medium. This population inversion grating persists as long as the frequencies of the two counter-rotating modes are sufficiently close (i.e. the intensity grating in the cavity moves slowly). Its contrast is all the weaker that the intensity grating moves rapidly with respect to the reaction time of the gain, that is to say that the frequency difference between counter-rotating waves is large compared with the inverse of the life time of the excited level.
French patent application FR 2905005 (THALES), describes a laser gyro comprising at least one ring optical cavity and a solid-state amplifying medium that are arranged in such a way that two so-called counter-rotating optical modes can coexist inside said optical cavity and can pass through the amplifying medium. The amplifying medium is coupled to an electromechanical device affording the amplifying medium a periodic translational motion along an axis substantially parallel to the direction of propagation of said optical modes.
Such a device makes it possible to modulate the longitudinal position of the active crystal about a mean position, so that the atoms of the crystal are in motion with respect to the nodes and to the antinodes of the interference pattern formed by the two counter-rotating modes, whatever the frequency difference between these two modes. Such a device makes it possible to decrease the contrast of the gain grating, and therefore its injurious effects on the gyrometry measurements while not modifying the length of the cavity. It also makes it possible to attenuate the effects of the backscattering induced by the amplifying medium. Finally, the device according to the invention potentially constitutes a device for dealing with the blind zone that can, as required, be substituted for or be complementary to the customary mechanical activation device.
Such a device does not make it possible to use a high activation frequency, necessary for use in the civil aviation sector or in a weapons system. Furthermore, the presence of the activation frequency of the amplifying medium in the useful band of the measurement frequencies causes disturbances to the optical intensities which may prevent proper observation of the output signals.